I can't say any more, it's meant to be a big secret.

我再也无可奉告了，这原本是个大秘密。

《柯林斯英汉双解大词典》

Things keep changing until he discovers a big secret, which becomes his motive for learning.

就这样，直到有一天，麦兜发现了一个天大的秘密，他由此产生了学习动力，一切开始转变。

youdao

It's no big secret that none of the traditional telecom services are generating the revenues we'd all hoped for.

这已经不是什么大秘密了，传统的电信服务商的收入都没有达到大家的期望。

youdao

It's a big secret. Everyone just wants to be cool.

一个大秘密。每个人都想很酷。

如果想去俱乐部 - SpeakingMax英语口语达人

That's the big secret about this city, I think.

我想这就是这座城市的一个大秘密吧。

如果想去俱乐部 - SpeakingMax英语口语达人

Rest assured, Jeremy will be letting us in on the big secret later this week.

ENGADGET

And there's one other big secret to this crust recipe: vodka, in equal proportion to the water.

NPR: A Thanksgiving Meal From The Test Kitchen

Let's be honest, it's no big secret that we're running out of dead dinosaurs to fuel our lives.

ENGADGET: Thin film coating makes everlasting energy a piezoelectric possibility